1. Field of the Invention
This invention relates to capacitance type displacement measuring instruments, and more particularly to improvements in a capacitance type displacement measuring instrument of vernier system, wherein a change in electric capacity between electrodes due to a relative displacement between two members movable relative to each other is detected on the basis of a change in phase of a detection signal, and a relative displacement between both members is measured from the change in the capacity.
2. Description of the Prior Art
In general, in a measuring instrument for measuring a length or the like of an article, when a movement value of articles movable relative to each other, such as a movement value of a measuring element to a main body or a movement value of a slider to a column, there have been know capacitance type displacement measuring instrument, in which a frame member holding a main scale is secured on one hand and a detector including an index scale is secured on the other hand, and a relative displacement value is read by an electrostatic method for example.
Of these capacitance type displacement measuring instruments, particularly as a capacitance type displacement measuring instrument of vernier system in which a change in electric capacity between electrodes due to a relative displacement between two members movable relative to each other is detected on the basis of a change in phase of a detection signal and a relative displacement between the both members is measured from the change in the capacity, there has been proposed one as disclosed in U.S. Pat. No. 3,068,457 for example. However, according to this U.S. Pat. No. 3,068,457, to obtain the vernier action, the electrodes provided on one of the members movable relative to each other are formed into complementary sine wave pattern electrodes and two pairs of the sine wave pattern electrodes are shifted in phase through 90 degrees from each other. Thus, such disadvantages have been presented that the members are increased in a dimension in the widthwise direction, and moreover, it is difficult to accurately form the two pairs of the sine wave pattern electrodes in a manner to be shifted in phase.
Furthermore, in Japanese Patent Laid-Open No. 94354/79 (corresponding to U.S. Pat. No. 4,420,754) for example, there has been proposed one in which a pair of electrodes are used and frequencies are shifted from each other in a signal processing circuit, to thereby attain the vernier action. However, such a disadvantage has been presented that the circuit arrangement becomes considerably complicated.